Gas chromatography involves vaporizing an analytic sample and injecting the vaporized sample onto a head of a chromatographic column. The vaporized sample is transported through the chromatographic column by a flow of an inert gas. A detector is then used to determine different components of the sample.
Various types of detectors can be used with gas chromatography equipment, and each type has particular advantages and disadvantages. One type of detector is a thermal conductivity detector, which for convenience is referred to hereinafter as a TCD. A TCD operates based on relative changes in the thermal conductivity of gas flowing through separate sample and reference cells of the TCD.
TCD's are valued for their relative simplicity, their relatively large linear dynamic range, and their general response to both organic and inorganic species. TCD's are also non-destructive, which permits collection of solutes after detection. TCD's are, however, often not as sensitive as other types of detectors. This limitation of existing TCD designs restricts the use of TCD's in some instances.
A TCD typically contains, in the sample and reference cells, pairs of electrically heated filaments whose temperature at constant electrical power depends upon the thermal conductivity of the surrounding gas. As carrier gas containing solutes passes through the cell, a change in the filament current occurs due to a change in the temperature of the filament. A comparison is made of current in a sample cell and current in a reference cell. A signal is generated from the measured difference. The resistances of the filament pairs are usually compared by incorporating them into two arms of a Wheatstone bridge circuit.
This Wheatstone bridge circuit is intended to amplify resistance changes due to analytes passing over the sample thermo-conductors, while disregarding changes in resistance that both sets of detectors produce due to flow rate fluctuations, etc. Two pairs of elements are used. One pair of elements is located in the flow of the effluent gas in the chromatographic column, and the other pair is located in the gas stream ahead of a sample injection chamber.
Direct-type designs provide favorable sensitivity, but at the expense of unfavorable stability and interference. By contrast, diffused-type designs are unfavorably diminished in sensitivity and response, but have favorably improved stability and interference characteristics. Accordingly, direct-type or diffused-type designs are selected for particular applications depending on which characteristics are important.
In practice, random gas flow fluctuations in gas chromatographic equipment adversely affect the performance of TCD. Such fluctuations affect direct-type designs more adversely than diffuse-type designs. In both cases, though, the quality of detected results deteriorates.
What is needed is a TCD design that has improved sensitivity, quick response time, good stability, and lower interference which enables it to be used in applications for which TCD have, heretofore, been unsuitable.